Chronic lymphocytic leukemia (CLL) comprises 10% of all adult hematologic malignancies, but 40% in those over 65 years of age. It arises through clonal expansion of CD5+ B lymphocytes and about 50% display somatic hypermutation of the variable region of the immunoglobulin heavy chain gene (IGHV). Patients exhibit great clinical heterogeneity in the course of their disease being indolent or aggressive, the latter requiring aggressive chemotherapeutic approaches. Currently, treatment decisions are based on clinical staging though with some inaccuracy. Recent biological studies have indicated that the mutational status of IGHV is prognostic of outcome. Due to the labor intensity and technical difficulties associated with the performance of this assay, recent efforts have focused on identifying surrogate markers for mutational status that can be easily assayed in a clinical setting by flow cytometry. Two such markers, CD38 and ZAP-70, have either proven not to be robust (CD38), or remains to be confirmed (ZAP-70). Our follow-up studies of the transcripts differentially expressed between unmutated and mutated CLL B cells by expression profiling have identified a more robust candidate, SEPT10, as a surrogate marker for IGHV mutational status than ZAP70. In the present Phase I application, we propose to develop a flow cytometric assay for Septin 10 expression in a panel of 50 CLL patients for which the mutational status will be determined, with the following specific aims: 1) Generate and confirm the specificity of Septin 10 antibodies and characterize the expression pattern by Western blotting, 2) Develop a flow cytometric assay for Septin 10 expression in mononuclear cells and whole blood, 3) Statistically correlate Septin 10 expression with IGHV mutational status, confirming the use of Septin 10 expression as a surrogate for IGHV mutational status. In Phase II, we plan to validate the correlation in a larger panel of 300 clinically well-characterized CLL patients, and evaluate the impact of this molecular marker in the risk stratification of CLL patients for treatment decisions. [unreadable] [unreadable] [unreadable] [unreadable]